This application claims the priority of European patent document 09 005 827.2, filed Apr. 27, 2009, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a master clock generation unit for satellite navigation systems, and to a master clock generation system.
A master clock generation unit known as Clock Monitor and Control Unit (CMCU) is a high performance 10.23 MHz master clock generation unit for satellite navigation systems. Two frequency synthesizers can select independently one of four signal inputs to which independent 10 MHz atomic clock frequency references can be provided as a basis for the master clock generation. One of the two synthesizer outputs is selected to provide a master clock for a payload and is distributed to up to four identical outputs. The output signal of the second synthesizer provides a hot redundant alternative for the master clock. A phase meter monitors the output phase of the active synthesizer against the hot redundant one.
Accordingly, the clock monitoring and control unit being part of the payload for satellite navigation systems generates a satellite's Master Timing Reference (MTR) signal based on input signals provided by atomic standards. The functional concept of a known master clock generation unit is shown in FIG. 1. The master clock generation unit or CMCU 100 derives an output reference frequency, namely a 10.23 MHz on-board Master Timing Reference (MTR), based on a set of four atomic frequency standards which are fed to frequency inputs 102, 104, 106 and 108. Each of the frequency inputs 102, 104, 106, 108 is connected to a respective matrix input 131, 132, 133, 134 of a 4×2 switching matrix 130. The switching matrix 130 enables selecting a nominal (primary) and a redundant (secondary) clock at a first and a second matrix output 135, 136. The switching matrix 130 is telecommanded via a controller 180. The nominal and the redundant clock at the first and the second matrix output 135, 136 are fed to a first and a second frequency synthesizer 140, 145. The frequency synthesizers 140, 145 are adapted to perform a frequency conversion according to different clock types: a Passive Hydrogen Maser (PHM) and a Rubidium Clock (RAFS). The respective synthesizer outputs 142, 147 are connected to a phase meter 170 and a switch 175. The phase meter 170 monitors the phase difference between the output signals of the frequency synthesizers 140, 145 and stores the results for later retrieval. One of the two synthesizers 140, 145 output signals is selected by the switch 175 to provide the 10.23 MHz reference frequency at four frequency outputs 202, 204, 206, 208 to the payload.
The above described master clock generation unit 100 is provided in identical manner in a master clock generation system 1 twice. Since both the nominal master clock generation unit (indicated by N in the left part of the master clock generation system) and the redundant master clock generation unit (indicated by R in the right part of FIG. 1) are identical for providing cold redundancy only the nominal CMCU-N is depicted with reference numerals.
The solution depicted in FIG. 1 and comprising a simplified switch matrix is currently used in Galileo satellite system. A major disadvantage of the described master clock generation unit is that, due to the different clock input frequencies, two frequency synthesizers 140, 145 are needed to perform phase comparison later on.
It is therefore an object of the present invention to provide a master clock generation unit for satellite navigation systems which can be built with cheaper manufacturing and part costs.
Furthermore, a master clock generation unit is to be provided which can provide a more accurate master timing reference.
These and other objects and advantages are achieved according to the invention by a master clock generation unit for satellite navigation systems, known as Clock Monitoring and Control Unit CMCU, which comprises a number of frequency inputs for receiving a respective atomic clock signal, each clock signal having a first or a second reference frequency, and a number of frequency converters, each of which has a converter input connected to one of the frequency inputs and a converter output. Each of the frequency converters is supplied with an offset frequency by one or a number of frequency synthesizers, the offset frequency being selected according to the first and the second reference frequency at the assigned frequency input for providing the same intermediate frequency at each converter output. A switching matrix is connected to each of the converter outputs for selecting one of the intermediate frequencies as a primary clock provided at a first matrix output, and another of the intermediate frequencies as a secondary clock provided at a second matrix output. A frequency generator which has a generating input connected to the first matrix output and is connected to a number of frequency outputs of the master clock generation unit, is adapted for deriving an output reference frequency from the primary clock and providing the output reference frequency at the number of frequency outputs. A phase meter having a first meter input connected to the first matrix output and a second meter input connected to the second matrix output is provided for determining a phase difference between the primary and the secondary clock to detect abnormal operation.
In another aspect of the invention a master clock generation system comprising two master clock generation units according to the invention for redundancy purposes is provided.
The master clock generation unit according to the invention guarantees high isolation between the input signals (i.e., the atomic clock signals). Furthermore, it accepts signals from different clock types which are operating on different frequencies in any combination. The master clock generation unit avoids any hard switching of the output signal, and therefore avoids phase jumps that might affect the remaining payload. A further advantage is an increase in resolution of the phase meter to provide more accurate results. At last, rapid changes of the reference frequency can be detected.
In a preferred embodiment the clock generation unit comprises up to four frequency inputs, two of them receiving the atomic clock signal having the first reference frequency and the other two receiving the atomic clock signal having the second reference frequency. The first reference frequency corresponds to 10.0028 MHz according to Phase Hydrogen Maser (PHM). The second reference frequency corresponds to 10.00 MHz according to Rubidium Clock (RAFS). The up to four input signals from the atomic clocks are down-converted to an intermediate frequency for further processing. To cope with the different frequencies of PHM and RAFS, it is preferred to provide two frequency converters assigned to signals according to PHM and two signals for RAFS which are driven by different frequency synthesizers. The frequency synthesizers are known as direct digital frequency synthesizers (DDS). By applying different offset frequencies to the frequency converters, both clock signals can be converted to the same intermediate frequency for further processing.
In a preferred embodiment each of the frequency converters comprises a first conversion stage for mixing the first reference frequency and the second reference frequency, respectively, with a reference frequency to a respective first and second pre-intermediate frequency, before tuning the first and the second pre-intermediate frequency to the same intermediate frequency provided at each of the converter outputs in a second conversion stage. It is preferred that the reference frequency is the output reference frequency provided by an oscillator of the frequency generator of the clock generation unit.
According to a further preferred embodiment one of the intermediate frequencies fed to the switching matrix can be chosen as a primary clock, and another can be chosen as a secondary clock, in the switching matrix. In the intermediate frequency domain, one of the clocks can be selected by the switching matrix as the primary clock from which to derive the master clock generation unit output signals. The second clock, kept for hot redundancy, will be routed through the switching matrix to the phase meter to be compared with a signal from the primary one.
According to a further embodiment of the invention, the frequency generator comprises a phase frequency detector for comparing the first clock with a signal derived from the second reference frequency, by a frequency division. Furthermore, an error signal generated by the frequency generator is used to control the reference frequency.
In a further embodiment of the invention, the phase meter arrangement comprises a number of phase meters that are connected to each of an assigned converter output for receiving the intermediate frequencies (i.e., down-converted reference frequencies), and connected to a reference clock signal as provided to the phase frequency detector of the frequency generator.
In still another embodiment of the invention, the phase meter stores a number of measurement samples in pre-determined time intervals for later retrieval. The phase meter determines the phase difference between the two input signals in programmable time intervals. Preferably, up to 1000 measurement samples can be stored inside the master clock generation unit for later retrieval by a data handling system. For rapid detection of any abnormal behavior of one of the two clocks, the phase meter preferably comprises a frequency discontinuity detector in addition.
The output reference frequency corresponds to 10.23 MHz.
The master clock generation unit according to the invention is powered by a primary power bus, especially a regulated bus providing a voltage of 50 V, from a spacecraft. Furthermore, it is useful if the switching matrix, the frequency synthesizers and the phase meter are commanded and supervised via a data interface which can be a standard serial interface. The master clock generation unit furthermore preferably provides discrete telemetry of the on/off status, voltage and current monitoring and thermistor telemetry.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.